feat: add setting to disable rainbow effect

[minghaoxia61-web]

Summary

Add a user setting to disable the animated rainbow border around the Continue input box.

Changes

• Add rainbowEffectEnabled to the UI configuration.
• Add an Appearance setting with a default value of true.
• Disable the rainbow animation when the setting is turned off.
• Preserve the existing default behavior and input functionality.
• Add regression tests for the component and settings update flow.

Verification

• Core TypeScript check passed.
• GUI TypeScript check passed.
• 9 new tests passed.
• Full GUI regression suite passed: 39 test files and 381 tests.
• ESLint passed for modified files.
• Prettier passed with the repository's CRLF line-ending configuration.
• git diff --check passed.

Closes #13175

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